Stubbing a function that does not belong to an object

stub-function.js

var jwt = function(x) {
  console.log('transform stuff');
}

function thingToBeTestedFactory(jwt) {
  return function(x) {
    return jwt(x);
  };
}

mocha.setup('bdd');
var expect = chai.expect;
    
describe('Thing', function() {
  it('should return a jwt-ed x', function() {
    var jwtStub = sinon.stub();
    var factory = thingToBeTestedFactory(jwtStub);
    var thingToBeTested = factory(5);
    expect(jwtStub.calledOnce).to.be.true;
  });
});

mocha.run();